A Controller Area Network (CAN bus) is a communications standard designed to allow microcontrollers and devices to communicate with each other in applications without a host computer. CAN-FD (flexible data-rate) is an extension of the CAN standard developed to improve the achievable bandwidths of CAN networks.
The bit rate requirements for CAN-FD networks are faster than those of standard CAN network, with the current requirement being 5 Mbps and higher rates planned for the future. In order to achieve the current 5 Mbps data rate, a maximum transmitted bit width (t_bit) of 200 ns is required.
FIG. 1 schematically illustrates the transmission of bits over a CAN bus. The logical state provided to the CAN driver, and which is to be transmitted over the CAN bus is illustrated at 110. The voltage level on the CAN bus channel (e.g. as might be seen on the CAN-H bus line) is illustrated at 120. The logical state output by a CAN receiver in response to the voltage level on the CAN bus channel is illustrated at 130. The voltage level on the CAN bus channel 120 is driven by the CAN driver in response to the logical state 110 to be transmitted. The duration of a particular state on the CAN bus is dependent not only on the logical state 110 provided to the CAN driver, but also on the rates at which the voltage level 120 on the CAN bus channel rises and falls.
The CAN standard (ISO 11898) requires symmetrical durations for bit states such that: Tbit_Tx≈Tbit_Bus≈Tbit_Rx. However, the physical layer open drain topology of a typical CAN driver means that the rise and fall times of the voltage level on the CAN bus channel 120 can differ significantly, resulting in asymmetrical durations for bit states. Specifically, the rise time of the voltage level on the CAN bus channel 120 is driven by the CAN PHY driver whilst the fall time is driven by passive elements. As a result, the fall time is typically much greater than the rise time. Increasing the rise time on the CAN bus channel would mean reducing the achievable bit rate (contrary to the desire to increase the achievable bit rates of CAN systems) whilst EMC compliancy constraints prohibit reducing the fall time on the CAN bus channel.